1. Field of the Invention
The invention relates to a reference piece for exact positioning of a workpiece in a prescribed operational position within the working zone of a wire spark-erosion machine, relative to the cutting wire of the machine, wherein outside the working zone, at least one planar base reference surface is formed on the mounting table of the machine which base reference surface extends in an x-direction and in a y-direction perpendicular to said x-direction at the level, or in the plane, of the lower limit line (base line) of the working zone with regard to a z-direction normal to said x- and y-directions.
2. Description of the Prior Art
Wire spark-erosion machines generally comprise an upper wire feed mechanism and a lower wire withdrawal mechanism, between which the cutting wire extends through the working zone, the direction of the cutting wire being usually the z-direction which may be vertical.
In the wire spark-erosion machines for which the invention is applicable a base reference surface is provided on the mounting table, outside the working zone, which base reference surface is employed for exact positioning of the workpiece which is to be acted upon by the cutting wire in the working zone. This base reference surface provides a reference surface for positioning the workpiece in the z-direction. Said surface is commonly a planar-ground surface of a support member which member rises from the mounting table in the z-direction and extends parallel to a lateral boundary of the working zone. The support rises far enough over the mounting table for the base reference surface to lie on the base line of the working zone, namely on the lower boundary of the working zone closest to the mounting table in the z-direction. There are known wire spark-erosion machines in which for each opposite lateral boundary of the working zone a support member with a respective base reference surface is provided. Other wire spark-erosion machines are known in which the support member has an angle shape, so that the base reference surface at the base line extends along two adjoining lateral boundaries of the working zone.
Wire spark-erosion machining of a workpiece with a cutting wire enables shaped pieces to be produced having extremely high dimensional precision and surface quality. Wire spark-erosion machining takes a relatively long operating time. Therefore it is important to minimize unproductive time in the machine which time is consumed in changing and positioning workpieces. For die-sinking spark-erosion machines, mechanisms are known which are provided with reference surfaces, which enable a workpiece which is to be worked on to be mounted outside the machine on holders provided with counter-reference surfaces. Then, in order to mount a new workpiece in the die sinking spark-erosion machine one needs merely to attach the holders, upon which the workpiece has been mounted, to the machine. The workpiece then is automatically and accurately positioned by the interaction of the counter-reference surfaces of the holders with the reference surfaces of the mechanisms remaining on the machine as shown in Federal Republic of Germany Pat. No. 2,646,951.
The wire spark-erosion technique has enjoyed good market success, even though it is slow and requires substantial capital expenditure. Its rapid acceptance as a machining method is attributable to a number of favorable characteristics, which have opened new avenues for the manufacture of complex parts with stringent criteria of dimensional tolerances and surface quality.
Very complex two-dimensional shapes can be produced by wire spark-erosion machining with the aid of a thin wire which is controlled by a computerized numerical control ("CNC") system relative to the working zone. The machining can be performed on any electrically conducting workpiece, without producing attendant stress-increasing forces and without limiting the hardness of the workpiece.
These properties have enabled workpieces meeting stringent requirements for dimensional tolerances and surface quality to be produced with the same machinery as used in finishing, smoothing, planishing, etc. Advantageously, the workpiece can be hardened prior to the wire spark-erosion operation. Because the wire spark-erosion operation is controlled by CNC, which is well suited for extending unmanned operation, the disadvantages of slow machining speed and high machinery cost can be minimized by high utilization of the machines.
A precondition for high utilization of the machines is a wide range of applicability, with the entire geometric and load-bearing capacity of the machine being utilizable without time-consuming work.
Because the wire spark-erosion technique is particularly suited for short-run, i.e., not mass production, manufacture of parts in the nature of tooling or prototypes, it is uneconomical to fabricate special clamping devices to precisely position each shape of workpiece on the machine.
The dimensions, weights, and volumes of the workpieces will vary over a broad range. Accordingly, it is important for the productivity of the machine to be able to engage and position individual workpieces of different shapes and sizes with a minimum of downtime.
By employing a general, flexible reference system it is possible to carry out the basic positioning steps outside the machine itself, while the machine is performing another task. For this purpose, a basic reference system must be present on the machine, which system is fixed in relation to the active cutting wire of the machine. Secondary reference levels for accurately positioning the new workpiece in the working zone of the machine can be set up on, e.g., a faceplate, with the aid of this basic reference system, wherein the coordinates can be given with respect to the basic system ("offset coordinates") and can be transmitted to the control system of the wire spark-erosion machine in that form. The requirements which such a general reference system must meet are, however, very stringent.
The first requirement applied to a reference system, which requirement aims at maintaining high flexibility and thus high utilization of the wire spark-erosion machine, is that the system must enable workpieces of widely varying shapes and weights to be held and accurately positioned in the working zone, with the use of a limited number of generally usable elements. Any limitation of the original capacity of the machine with respect to geometry or weight will have a negative effect on utilization.
Because wire spark-erosion machines can machine complex geometries with stringent requirements for dimensional tolerances and surface quality, subsequent machining of workpieces is minimized or is completely obviated. Accordingly, despite low actual machining rates, the throughput time of a workpiece can be kept short by means of relatively long continuous unmanned operation. A prerequisite for such machining to final dimensions with close tolerances is that one be able to engage and fix the workpiece with a reference system which has high reproducibility and accuracy. In this connection, attention must be paid to the load-bearing capacity of the reference system, because elastic positional errors, e.g. due to elastic angular deviations, can themselves often exceed the prescribed dimensional tolerances for the finished workpiece.
As the need to machine conical surfaces at high cone angles becomes more frequent, the reference scale in the z-direction (vertical direction) of the machine becomes increasingly important with regard to the machining tolerance. A positional error of 0.02 mm in the z-direction gives rise to an error in the x-direction (which may be horizontal) of 0.01 mm when the cone angle is 30.degree., for example.
Further, with a general reference system economic aspects must be taken into account. For technically and economically feasible wire spark-erosion machining, the entire application range of the machine must be accessible with the use of a minimum number of reference elements. Modular construction, symmetry of design, and multiple applicability (multiple interchangeability) of the available reference elements all facilitate such technical and economic aspects. Because the electrodes employed in die-sinking spark-erosion machining can be advantageously manufactured on wire spark-erosion machines, compatibility of the auxiliary hardware of the two machines will also result in higher utilization of the available reference elements.
Moreover, wire spark-erosion and die-sinking spark-erosion machining are often carried out in processes involving several other machines. Accordingly, in order to achieve high utilization of available reference elements the reference elements should be capable of meeting the requirements of a number of machining processes, such as grindingm milling, turning, and facing, as well as wire spark-erosion and die-sinking spark-erosion, and should be easily capable of being measured on an ordinary faceplate. In addition, the base point references of the various machines should be capable of being coordinated with current CNC technology, which affords advances in programmed operation.
Examples of such a general auxiliary hardware system with reference levels fixed to the machine or adjustable outside the machine, which system does not limit the application range of the wire spark-erosion machine, are found in U.S. patent application Ser. No. 06/708,820, now U.S. Pat. No. 4,656,326.
There are also machines on the market which are provided with fixed base reference points on the z-axis and at angles "A" and "B", with such reference system not being applicable with general auxiliary hardware. The angle "A" is an angle in a plane which is perpendicular to the x-axis, and the angle "B" is an angle in a plane perpendicular to the y-axis. As a rule, these machines are equipped with a reference table in the form of a frame structure; or they may be equipped with an angle table or merely support rails which rails are disposed around the horizontal working zone of the machine and outside of said zone (see FIGS. 1 and 2).
These configurations use their upper limiting surface to define an accurate base reference surface in the vertical (z-axis) direction, as well as along angles "A" and "B". Invariably the base reference surface of the z-axis coincides with the lower limit of the vertical working zone of the machine, which lower limit is called the "base line". Accordingly, this base reference surface is employed as a direct support surface for large workpieces, and as an attachment surface for clamping means for such workpieces.
Another characteristic of these machines is that they lack usable base reference surfaces for the x- and y-axes; further, that there is no angle "C", which is an angle in a plane perpendicular to the z-axis.
On such machines, for each engagement of a workpiece the operator must find and fix the reference points on the x- and y-axes and along the angle "C", with the aid of electrical measuring devices provided on the machine. These adjustments are manual and time-consuming, and therefore result in a loss of machining time on the machine.
Also, distinct problems arise in engaging small and medium sized workpieces, which cannot be affixed directly to the reference table of the machine in order to fix them in the z-axis. When such workpieces are to be fixed within the horizontal working zone of the machine, errors in positioning often occur, on the z-axis and along angles "A" and "B". Particularly on the z-axis, such positioning errors are difficult and time-consuming to detect by using the measuring devices present on the machine. The consequence is tolerance errors, particularly when the "U"- and "V"-axes of the machine are used for conical machining.
In order to increase the flexibility of these machines, particularly when machining small or medium sized workpieces, and to exploit the possibilities of CNC technology with fixed reference points for the workpiece, with the machine programming then being executable from said points, this type of machine needs to be equipped with a holding system which is flexible to apply and which can accurately fix workpieces of a wide range of sizes (large, medium, and small). Such a holding system should at least define a reference system on the x-, y-, and z-axes, at an angles "A", "B", and "C", which system can be related in a fixed manner to the control system of the machine and can provide the base reference system for said control system.